1. Field of the Invention
The present invention relates to a vertical GaN light emitting diode and a method for manufacturing the same, and more particularly to a vertical GaN light emitting diode, from which an insulating sapphire substrate with low thermal conductivity is removed and in which a conductive substrate such as a silicon substrate is installed so as to improve the luminance and the reliability of the diode, and a method for manufacturing the vertical GaN light emitting diode.
2. Description of the Related Art
Generally, light emitting diodes (LEDs) are semiconductor elements, which emit light based on the recoupling of electrons and holes, and are widely used as various types of light sources in optical communication and electronic equipment. GaN serves as a compound for manufacturing blue-light emitting diodes.
Frequency (or wavelength) of light emitted from the light emitting diode is functionally related to a band gap of a semiconductor material to be used. When the band gap is small, photons with low energy and a longer wavelength are generated. In order to generate photons with a shorter wavelength, there is required a semiconductor material with a broader band gap.
For example, AlGaInP commonly used in a laser beam emits light corresponding to visible red light (approximately 600˜700 nm). On the other hand, silicon carbide (SiC) and Group III nitride semiconductor materials such as gallium nitride (GaN) with a comparatively broad band gap emit light corresponding to visible blue light or ultraviolet rays. A short wavelength LED has an advantage in increasing a storage space of an optical storage (approximately 4 times as large as that of a general LED emitting red light).
The same as other Group III nitride semiconductor materials for emitting blue light, there is no practical technique for forming a bulk single crystal made of GaN. Accordingly, there is required a substrate suitable for growing a GaN crystal thereon. Sapphire, i.e., aluminum oxide (Al2O3), is typically used as such a substrate for growing the GaN crystal thereon.
However, a sapphire substrate has an insulating property, thus limiting the structure of a GaN light emitting diode. With reference to FIG. 1, the structure of a conventional GaN light emitting diode is will be described in detail.
FIG. 1 is a cross-sectional view of a conventional GaN light emitting diode 10. The GaN light emitting diode 10 comprises a sapphire substrate 11 and a GaN light emitting structure 15 formed on the sapphire substrate 11.
The GaN light emitting structure 15 includes an n-type GaN clad layer 15a, an active layer 15b formed to have a multi-quantum well structure, and a p-type GaN clad layer 15c. Here, the n-type GaN clad layer 15a, the active layer 15b and the p-type GaN clad layer 15c are sequentially formed on the sapphire substrate 11. The light emitting structure 15 may be grown on the sapphire substrate 11 using MOCVD (metal-organic chemical vapor deposition), etc. Here, in order to improve the lattice matching of the light emitting structure 15 and the sapphire substrate 11, a buffer layer (not shown) made of AlN/GaN may be formed on the sapphire substrate 11 before the growing of the n-type GaN clad layer 15a. 
The p-type GaN clad layer 15c and the active layer 15b are removed at designated portions by dry etching so as to selectively expose the upper surface of the n-type GaN clad layer 15a. An n-type contact 19 is formed on the exposed upper surface of the n-type GaN clad layer 15a, and a p-type contact 17 is formed on the upper surface of the p-type GaN clad layer 15c. A designated voltage is applied to the n-type contact 19 and the p-type contact 17. Generally, in order to increase a current injection area while not negatively affecting luminance, a transparent electrode 16 may be formed on the upper surface of the p-type GaN clad layer 15c before the forming the p-type contact 17 on the p-type GaN clad layer 15c. 
As described above, since the conventional GaN light emitting diode 10 uses the insulating sapphire substrate 11, the two contacts 17 and 19 are formed on the sapphire substrate so that the contacts 17 and 19 are nearly horizontal with each other. Accordingly, when a voltage is applied to the conventional GaN light emitting diode 10, a current flows from the n-type contact 19 to the p-type contact 17 via the active layer 15b in a horizontal direction. Since a forward voltage (Vf) of the light emitting diode 10 is increased due to this narrow current flow, the current efficiency of the light emitting diode 10 is lowered and an electrostatic discharge effect is weak.
Further, the conventional GaN light emitting diode 10 emits a great amount of heat in proportion to the increase of the, current density. On the other hand, the sapphire substrate 11 has low thermal conductivity, thus not rapidly dissipating heat. Accordingly, mechanical stress is exerted between the sapphire substrate 11 and the GaN light emitting structure 15 due to the increased temperature, thus causing the GaN light emitting diode 10 to be unstable.
Moreover, in order to form the n-type contact 19, a portion of the active layer 15b with a size at least larger than that of the contact 19 to be formed must be removed. Accordingly, a light emitting area is reduced, and the luminous efficiency according to the luminance relative to the size of the diode 10 is lowered.
Therefore, there are required a GaN blue light emitting diode with improved luminance and reliability thereof, in which the above-described problems arising from the use of the sapphire substrate required to grow a GaN single crystal thereon are removed, and a method for manufacturing the GaN blue light emitting diode.